Scientists find link between tropical storms and decline of river deltas

A change in the patterns of tropical storms is threatening the future of the Mekong River delta in Vietnam, indicating a similar risk to all of the world’s major river deltas.

Professor Dan Parsons and Dr Chris Hackney, of the University of Hull’s Energy and Environment Institute, led the river surveys which underpinned the study, funded by the UK Natural Environment Research Council (NERC) and conducted by an international team of scientists.

The research, published in the Journal Nature, shows that fewer tropical storms have been hitting the Mekong catchment in recent years, resulting in much less mud and sand reaching the delta.

This threatens the delta’s sustainability in the medium and longer term due to the adverse impacts on flooding and reduced agricultural productivity.

The Mekong, the world’s third largest river delta, is home to 20 million people and a large agricultural area dominated by rice. As such, it is vital to the economy and food security of Vietnam and the region.

Deltas are landforms made from sediment washed into rivers and carried downstream.  The sediment builds up where the river meets slow moving or still water, such as seas or lakes.

These results are very significant. The sediment load is already declining as a result of upstream damming and other human impacts such as sand mining. The impact of reduced sediment supply through a reduction in tropical storm influence will be like a ‘double whammy’ for the delta. Professor Dan Parsons,
Director of the Energy and Environment Institute

Deltas naturally subside under their own weight, so a constant flow of new deposits is vital to offset these changes and prevent flooding.

Scientists found that changes in the behaviour of cyclones mean less sediment is running into rivers upstream of the Mekong delta, starving it of the sediments needed to guard against flooding. 

Dr Hackney added: “Our findings are important in terms of Mekong delta sustainability into the future.

“Climate models predict that tropical cyclones will get stronger as the climate warms but storm tracks will also move further to the North-East and away from the Mekong’s catchment, exacerbating the problems of a reduced sediment supply.”

The research has global implications as other major rivers such as the Ganges (India/Bangladesh), Yangtze (China) and Mississippi (USA) have catchments that are regularly struck by tropical storms. 

Some 500 million people live and work in the world’s major river deltas. This study indicates that changes in storm climatology, even in the river catchments far upstream of the deltas themselves, must also be considered when evaluating their future vulnerability to sea-level rise.

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